A. A CMOS checksum is generated by adding all of
the bytes (or bits) in the CMOS one after the other. That is, byte one is
added to byte 2, byte 3 is added is to the sum of bytes 1 and 2, etc. The
carry bits are dropped. The result (checksum) is stored in the CMOS. During
the boot-up process or POST (Power-On Self Test) a checksum is generated
by the BIOS from the CMOS and compared to the one saved the last time the
CMOS Setup was run or the BIOS defaults were loaded. If the two numbers
don't agree it is an indication that the data in the CMOS has been corrupted
(one or more bits in the CMOS changed when it/they weren't supposed to) and
a checksum error is issued by the BIOS ("CMOS checksum invalid, " "CMOS
invalid," and relate error, "CMOS battery low"). Causes
include:

A bad battery.

A battery that has become discharged (the computer has
been off a very long time).

A disconnected battery.

Insertion of an expansion board in such a manner (cock-eyed)
as to short-out the bus (even if the computer is off, which it should be)